Many vertebrates, including man, lack the ability to manufacture a number of amino acids and therefore require these amino acids preformed in the diet. These are called essential amino acids. Human food and animal feed, derived from many grains, are deficient in essential amino acids, such as lysine, the sulfur amino acids methionine and cysteine, threonine and tryptophan. For example, in corn (Zea mays L.) lysine is the most limiting amino acid for the dietary requirements of many animals. Soybean (Glycine max L.) meal is used as an additive to corn-based animal feeds primarily as a lysine supplement. Thus, an increase in the lysine content of either corn or soybean would reduce or eliminate the need to supplement mixed grain feeds with lysine produced via fermentation of microbes. Furthermore, in corn the sulfur amino acids are the third most limiting amino acids, after lysine and tryptophan, for the dietary requirements of many animals. The use of soybean meal, which is rich in lysine and tryptophan, to supplement corn in animal feed is limited by the low sulfur amino acid content of the legume. Thus, an increase in the sulfur amino acid content of either corn or soybean would improve the nutritional quality of the mixtures and reduce the need for further supplementation through addition of more expensive methionine.
Efforts to improve the sulfur amino acid content of crops through plant breeding have met with limited success on the laboratory scale and no success on the commercial scale. A mutant corn line which had an elevated whole-kernel methionine concentration was isolated from corn cells grown in culture by selecting for growth in the presence of inhibitory concentrations of lysine plus threonine [Phillips et al., Cereal Chem., (1985), 62, 213-218]. However, agronomically-acceptable cultivars have not yet been derived from this line and commercialized. Soybean cell lines with increased intracellular concentrations of methionine were isolated by selection for growth in the presence of ethionine [Madison and Thompson, Plant Cell Reports, (1988), 7, 472-476], but plants were not regenerated from these lines.
Lysine, threonine, methionine, cysteine and isoleucine are amino acids derived from aspartate. One approach to increasing the nutritional quality of human foods and animal feed is to increase the production and accumulation of specific free amino acids via genetic engineering of the biosynthetic pathway that leads from aspartate to lysine, threonine, methionine, cysteine and isoleucine. However, few of the genes encoding enzymes that regulate this pathway in plants, especially corn, soybeans and wheat, are available. Alteration of the activity of enzymes in this pathway could lead to altered levels of lysine, threonine, methionine, cysteine and isoleucine. For instance, recombinant DNA and gene transfer technologies have been applied to alter enzyme activity at key steps in the amino acid biosynthetic pathway. The introduction into plants of a feedback-regulation-insensitive dihydrodipicolinic acid synthase (“DHDPS”) gene, which encodes an enzyme that catalyzes the first reaction unique to the lysine biosynthetic pathway, has resulted in an increase in the levels of free lysine in the leaves and seeds of those plants (Falco, U.S. Pat. No. 5,773,691; Glassman, U.S. Pat. No. 5,258,300). Also, expression in plants of a bacterial lysC gene with aspartate kinase activity has resulted in an increase in threonine content of the seed (Karchi, et al. The Plant J. 3:721-727 (1993); Galili, et al., European Patent Application No. 0485970). However, expression of the lysC gene results in only a 6-7% increase in the level of total threonine or methionine in the seed; thus, feed containing lysC transgenic seeds still requires amino acid supplementation.
The organization of the pathway leading to biosynthesis of lysine, threonine, methionine, cysteine and isoleucine indicates that over-expression or reduction of expression of several genes encoding key regulatory enzymes of the pathway in corn, soybean, wheat and other crop plants could be used to alter levels of these amino acids in human food and animal feed. For example, methionine, along with threonine, lysine and isoleucine, are amino acids derived from aspartate. The first step in the pathway is the phosphorylation of aspartate by the enzyme aspartate kinase (Tang et al. (1997) Plant Mol Biol 34:287-293; Frankard et al. (1997) Plant Mol. Biol 34:233-242), and this enzyme has been found to be an important target for regulation of the pathway in many organisms. The aspartate family pathway is also believed to be regulated at the branch-point reactions. For methionine the reduction of aspartyl β-semialdehyde by homoserine dehydrogenase (HDH) may be an important point of control. Some aspartate kinases only carry aspartate kinase activity, in which case they are referred to as monofunctional, whereas there are bifunctional proteins found in bacteria and plants that carry both aspartate kinase and homoserine dehydrogenase enzymatic activities in two separate domains on one polypeptide. The first committed step to methionine, the production of cystathionine from O-phosphohomoserine and cysteine by cystathionine γ-synthase (CS), appears to be an important point of control of flux through the methionine pathway [Giovanelli et al., Plant Physiol., (1984), 77, 450-455]. The final step in methionine biosynthesis is catalyzed by the enzyme 5-methyltetrahydropteroyltriglutamate-homocysteine methyltransferase, also known as methionine synthase. Accordingly, availability of nucleic acid sequences encoding all or a portion of aspartate kinase would facilitate development of nutritionally improved crop plants.